1. Field of the Disclosure
The present disclosure relates to a method for enhancing recovery of oil from an oil-bearing carbonate/dolomite formation.
2. Description of the Related Art
Many reservoirs from which oil and gas are produced are not homogeneous in the geologic properties (e.g. porosity and permeability). In fact, many of such reservoirs, especially those consisting of carbonate type of rocks (e.g. limestone and dolomite) are naturally and significantly fractured. In addition, in carbonate reservoirs the rock matrix is often naturally fractured.
Carbonate reservoirs consist of two distinct elements: a fracture network and a microporous matrix. The fracture network is a series of interconnected cracks that can transmit fluids easily (very high permeability), but make up only a small fraction of the total porosity. The microporous matrix consists of the oil-bearing porous rock that typically exhibits much lower permeability than the fracture network but has the bulk of the total porosity of the reservoir. Hydrocarbon production is normally less efficient in fractured reservoirs. During primary production, the natural reservoir pressures to produce the oil in place will quickly decrease and more than 90% of the original oil in still left in place trapped in mostly in microporous matrix. Similarly, conventional methods of secondary recovery fail to displace substantial volumes of “left-in-place” oil.
Conventional waterflooding techniques exhibit relatively low efficiency in highly fractured reservoirs. Waterflooding in these reservoirs is characterized by early water breakthrough and rapidly increasing water-oil ratios to an uneconomic level. The injected water tends to travel only through the fractures and not interact with the oil trapped in rock matrix. The injected water cannot penetrate into the matrix and thereby displace and recover oil trapped in the microporous matrix. The injected water tends to recover only the oil left behind in the fracture system following primary production. The injected water has little or no interaction with oil trapped in the microporous matrix as the matrix is not water-wet. The microporous matrix does not spontaneously imbibe or absorb water and the injected water is most mobile through high-permeability fracture network and not the low-permeability microporous matrix where most of oil is trapped.
One approach to increase the penetration of a water phase into the microporous matrix has been to add a surfactant to the water to modify the wettability of the carbonate formation from oil-wet to water-wet. Previous research and field experience has demonstrated that including a low concentration of the properly selected surfactant to the water will reduce the interfacial tension and also create now a water-wet condition in the area near the fracture face. With this altered condition, the aqueous phase then penetrates some distance into the microporous matrix and thereby pushes out some of the oil that was within the pore spaces. In this countercurrent imbibition process, the oil that is displaced from the matrix then moves into the fracture network. Once pushed into the fracture network, the oil can be moved easily to a production well. In a countercurrent imbibition process, with or without the addition of a water-wetting surfactant, the rate of oil recovery is dependent upon the capillary pressure characteristics of the porous rock matrix. That is, the imbibition process is essentially unaffected by conventional techniques for controlling field operations, such as selecting pressures and flow rates.
Techniques for using surfactants in oil recovery in carbonate formation are disclosed by G. Hirasaki and D. L. Zhang in “Surface Chemistry of Oil Recovery from Fractured, Oil-Wet Carbonate Formations” (2000); by Austad and Standes in “Spontaneous Imbibition of Water into Oil-Wet Carbonates”, Journal of Petroleum Science and Engineering, vol. 39, pp. 363-376, (2003); by W. W. Weiss in “Artificial Intelligence Used to Evaluate 23 Single-Well Surfactant Soak Treatments”, SPE Reservoir Evaluation & Engineering, June 2006; U.S. Pat. Nos. 2,792,894; 4,364,431; 4,842,065; 5,247,993; and U.S. Published Patent Application No. 2007/0215347 A1.
Another approach to increase the penetration of a water phase with the matrix blocks containing trapped oil has been to inject foam under pressure into the oil-bearing carbonate formation. The foam is formed by aeration of a mixture of surfactant and water. The foam exhibits high apparent viscosity, which reduces the mobility of the water/surfactant solutions into large fractures or high-permeability zones effectively closing them off and/or providing a barrier to entry. With the altered condition, a subsequently introduced foam and/or a surfactant solution generated by the collapse of the original foam is able to penetrate into the microporous matrix acting to reduce the interfacial surface tension (IFT) between water and the oil in the microporous matrix thereby mobilizing the oil.
A problem with the use of foams for mobility control is the inherent short life of the foams. The foams dissipate relatively quickly diminishing their efficacy in blocking large fractures and any enhancement in oil recovery.
Another difficulty in recovering oil from fractured reservoirs or carbonate formations is compromise of internal pressure from adjacent gas-bearing formations referred to as “gas caps.” As oil is recovered from oil-bearing formations, pressure loss is encountered and gas seeps or permeates into the oil-bearing formations from the adjacent gas-bearing formations. The infiltration of gas pushes downward oil in the oil-bearing formations making the oil more difficult to extract.
It would be desirable to have a method for enhancing the transport and permeation of aqueous fluids through the microporous matrix of oil-bearing carbonate formations while substantially reducing or preventing transport and permeation through the natural fracture network. It would also be desirable to have a method for substantially reducing or preventing invasive permeation of gas from adjacent formations through the fracture network. It would also be desirable to have a method for enhancing the recovery of oil from oil-bearing carbonate formations and reservoirs.